1. Field of Invention
This invention relates generally to electrical interconnection systems and more specifically to improved signal integrity in interconnection systems, particularly in high speed electrical connectors.
2. Discussion of Related Art
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) which are then connected to one another by electrical connectors. A traditional arrangement for connecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected through the backplane by electrical connectors.
Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased significantly in recent years. Current systems pass more data between printed circuit boards and require electrical connectors that are electrically capable of handling the increased bandwidth.
As frequency content increases, there is a greater possibility of energy loss. Energy loss can be attributed to impedance discontinuities, mode conversion, leakage from imperfect shielding, or undesired coupling to other conductors (crosstalk). Therefore, connectors are designed to control the mechanisms the enable energy loss. Conductors composing transmission paths are designed to match system impedance, enforce a known propagating mode of energy, minimize eddy currents, and isolate alternate transmission paths from one another. One example of controlling energy loss is the placement of a conductor connected to a ground placed adjacent to a signal contact element to determine an impedance and minimize energy loss in the form of radiation.
Cross-talk between distinct signal paths can be controlled by arranging the various signal paths so that they are spaced further from each other and nearer to a shield. Thus, the different signal paths tend to electromagnetically couple more to the shield and less with each other. For a given level of cross-talk, the signal paths can be placed closer together when sufficient electromagnetic coupling to the ground conductors is maintained.
Although conductors are typically isolated from one another with shields are typically made from metal components, U.S. Pat. No. 6,709,294 (the '294 patent), which is assigned to the same assignee as the present application and which is hereby incorporated by reference in its entirety, describes making an extension of a shield plate in a connector from conductive plastic.
Electrical connectors can be designed for single-ended signals as well as for differential signals. A single-ended signal is carried on a single signal conducting path, with the voltage relative to a common reference conductor being the signal.
Differential signals are signals represented by a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, the two conducting paths of a differential pair are arranged to run near each other. No shielding is desired between the conducting paths of the pair but shielding may be used between differential pairs.
One example of a differential pair electrical connector is shown in U.S. Pat. No. 6,293,827 (the '827 patent), which is assigned to the assignee of the present application. The '827 patent discloses a differential signal electrical connector that provides shielding with separate shields corresponding to each pair of differential signals. U.S. Pat. No. 6,776,659 (the '659 patent), which is assigned to the assignee of the present application, shows individual shields corresponding to individual signal conductors. Ideally, each signal path is shielded from all other signal paths in the connector. Both the '827 patent and the '659 patents are hereby incorporated by reference in their entireties.
U.S. Pat. No. 6,786,771, (the '771 patent), which is assigned to the assignee of the present application and which is hereby incorporated by reference in its entirety, describes the use of lossy material to reduce unwanted resonances and improve connector performance, particularly at high speeds (for example, signal frequencies of 1 GHz or greater, particularly above 3 GHz).